Resubmit transactions that were invalidated by rolled back blocks

Suppose our mempool contains transaction `A`, and we apply a block that contains a transaction `B` that invalidates `A` (maybe `A` and `B` share an input). If we now switch to a different fork that does _not_ contain `B`, the mempool will be revalidated, but we will not reintroduce `A`, and it will be lost (unless the wallet that submitted it resubmits it).